Our laboratory is engaged in studies of the retinal circulation in living monkeys and eventually man. The techniques involve measurement of retinal blood flow from densitometry curves constructed from fluorescein retinal photographs. Thus, we have been able to show that vascular volume and retinal flow increase with rise in carbon dioxide blood levels, and fall with decrease in CO2. Increase in arterial oxygen causes constriction of vessels with fall in flow. Conversely, low oxygen supply dilates the retinal vessels and flow increases. The retinal vessels in these experiments are regulated similarly to those of the brain.